Process for producing electrolyte material for polymer electrolyte fuel cells, and membrane-electrode assembly for polymer electrolyte fuel cells

ABSTRACT

A membrane-electrode assembly for polymer electrolyte fuel cells which is excellent in water repellency and gas diffusivity and which exhibits a high output power density, can be obtained by using, as an electrolyte material for polymer electrolyte fuel cells, a fluoropolymer obtained by contacting a fluoropolymer which is excellent in gas diffusivity and which has alicyclic structures in its main chain and further has sulfonic acid groups, with fluorine gas for fluorination to increase water repellency and stabilize the molecule ends. The above fluoropolymer is preferably made of a copolymer comprising repeating units based on the following monomer A and repeating units based on the following monomer B (wherein Y is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, p is 0 or 1, and n is an integer of from 1 to 12):
         Monomer A: A perfluoromonomer which gives a polymer having repeating units containing cyclic structures in its main chain by radical polymerization   Monomer B: CF 2 ═CF—(OCF 2 CFY) m —O p —(CF 2 ) n —SO 3 H.

TECHNICAL FIELD

The present invention relates to a membrane-electrode assembly forpolymer electrolyte fuel cells and a polymer electrolyte materialtherefor.

BACKGROUND ART

Attention has been drawn to a hydrogen-oxygen fuel cell as a powergenerating system which presents substantially no adverse effects on theglobal environment because in principle, its reaction product is wateronly. Polymer electrolyte fuel cells were once mounted on spaceships inthe Gemini project and the Biosatellite project, but their powerdensities at the time were low. Later, more efficient alkaline fuelcells were developed and have dominated the fuel cell applications inspace including space shuttles in current use.

Meanwhile, with the recent technological progress, attention has beendrawn to polymer fuel cells again for the following two reasons: (1)Highly ion-conductive membranes have been developed as polymerelectrolytes and (2) it has been made possible to impart extremely highactivity to the catalysts for use in gas diffusion electrodes by the useof carbon as the support and an ion exchange resin coating.

Accordingly, extensive studies have been conducted on a process forproducing an electrode/polymer electrolyte membrane assembly(hereinafter referred to simply as an assembly) for polymer electrolytefuel cells.

The polymer electrolyte fuel cell which is presently being studied, hasa low operation temperature of from 50 to 120° C., and therefore isdefective in that exhaust heat can hardly be utilized effectively fore.g. an auxiliary power for electrolyte fuel cells. For the purpose ofcompensating such a defect, polymer electrolyte fuel cells are requiredto have a particularly high output power density. Further, as an objectfor the practical use, it is required to develop an assembly which cangive a high energy efficiency and a high output power density even underan operational condition of high utilization of fuel and air.

Under such operational conditions of a low operation temperature andhigh utilization of gas, especially in a cathode where water is formedby the cell reaction, clogging of an electrode porous body due tocondensation of water vapor (flooding) is likely to occur. Accordingly,in order to obtain long-term stable properties, it is necessary tosecure water repellency of the electrode so as not to cause flooding.This is particularly important for a polymer electrolyte fuel cell whichgives a high output power density at a low temperature.

In order to secure the water repellency of the electrode, it iseffective to reduce the ion exchange capacity of an ion exchange resinwhich coats a catalyst in the electrode, namely, to use an ion exchangeresin with a low content of ion exchange groups. However, in such acase, the water content of the ion exchange resin tends to be low,whereby the electroconductivity decreases, and the cell performancedecreases. Further, the gas permeability of the ion exchange resindecreases, whereby the supply of the gas to be supplied to the catalystsurface via the ion exchange resin coating will be slow. Therefore, thegas concentration in the reaction site decreases and voltage lossincreases. Namely, the concentration overvoltage increases, and theoutput power decreases.

Accordingly, it has been attempted to use a resin having a high ionexchange capacity as an ion exchange resin which coats a catalyst, andin addition, to incorporate a fluororesin, such as apolytetrafluoroethylene (hereinafter referred to as PTFE), atetrafluoroethylene (hereinafter referred to as TFE)/hexafluoropropylenecopolymer or a TFE/perfluoro(alkyl vinyl ether) copolymer, or the like,as a water repellent agent, in the electrode, especially in the cathode,thereby to suppress flooding (see, for example JP-A-5-36418). In thisspecification, “an A/B copolymer” means a copolymer comprising repeatingunits based on A and repeating units based on B.

However, if the amount of the above water repellent agent in theelectrode is increased so as to have sufficient water repellency, theelectrical resistance of the electrode increases, because the abovewater repellent agent is an insulator. Further, there is a problem thatthe gas permeability decreases due to an increase of the thickness ofthe electrode, and the output power rather decreases. In order tocompensate the decrease of electroconductivity of the electrode, it isnecessary to increase the electroconductivity of e.g. a carbon materialas a carrier for the catalyst or the ionic conductivity of the ionexchange resin which coats the catalyst. However, it is difficult toobtain an electrode which satisfies both sufficient electroconductivityand sufficient water repellency, and thus, it was not easy to obtain apolymer electrolyte fuel cell with a high output power and long-termstability.

Further, a method of mixing fluorinated pitch (see, for example,JP-A-7-211324) and a method of treating a catalyst carrier byfluorination (see, for example, JP-A-7-192738) have been also proposed,but there is a problem that the surface of the catalyst can notuniformly be coated with an ion exchange resin. Still further, a methodof letting the water repellency have a gradient in the thicknessdirection of the electrode (see, for example, JP-A-5-251086 andJP-A-7-134993) has been proposed, but the production process tends to becumbersome.

In order to increase the output power of the fuel cell, it is necessarythat the ion exchange resin in the electrode has high gas permeabilityand high electroconductivity, and such an ion exchange resin preferablyhas a high concentration of exchange groups and a high water content.However, if such an ion exchange resin having a high concentration ofexchange groups is used, flooding tends to occur, and the output powertends to decrease during long-term use though the initial output powerbecomes high by virtue of the high permeability of the fuel gas andelectroconductivity.

In order to solve such problems, the present inventors proposed aperfluoropolymer having alicyclic structures in its main chain(JP-A-2002-260705). Although improvement was made as compared with alinear perfluoropolymer having sulfonic acid groups usually used for apolymer electrolyte fuel cell, such a perfluoropolymer having alicyclicstructures in its main chain was not sufficient for durability or thelike if it was exposed to more severe conditions.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide apolymer electrolyte fuel cell which can maintain a high output power fora long term, as it has a cathode having high electroconductivity,containing an ion exchange resin with high gas permeability and havinghigh water repellency which can be maintained even in a long term use.

The present invention provides a process for producing an electrolytematerial for polymer electrolyte fuel cells which is made of afluoropolymer having alicyclic structures in its main chain and furtherhaving sulfonic acid groups, which comprises a step of obtaining afluoropolymer having alicyclic structures in its main chain and furtherhaving —SO₂F groups by radical polymerization and then contacting thefluoropolymer with fluorine gas and a step of converting the —SO₂Fgroups to sulfonic acid groups; and a process for producing a liquidcomposition, which comprises dissolving or dispersing the electrolytematerial obtained by the above process in a —OH group-containing organicsolvent.

Further, the present invention provides a process for producing amembrane-electrode assembly for polymer electrolyte fuel cells whichcomprises a membrane-form polymer electrolyte which is made of afluoropolymer having alicyclic structures in its main chain and furtherhaving sulfonic acid groups, a cathode disposed on one side of theelectrolyte and an anode disposed on the other side of the electrolyte,wherein the membrane-electrode assembly is produced via a step ofobtaining a fluoropolymer having alicyclic structures in its main chainand further having —SO₂F groups by radical polymerization and thencontacting the fluoropolymer with fluorine gas, a step of converting the—SO₂F groups to sulfonic acid groups, and a step of forming the abovefluoropolymer into a membrane-form.

Furthermore, the present invention provides process for producing amembrane-electrode assembly for polymer electrolyte fuel cells whichcomprises a membrane-form polymer electrolyte which is made of afluoropolymer having alicyclic structures in its main chain and furtherhaving sulfonic acid groups, a cathode disposed on one side of theelectrolyte and an anode disposed on the other side of the electrolyte,wherein the membrane-electrode assembly is produced via a step ofobtaining a fluoropolymer having alicyclic structures in its main chainand further having —SO₂F groups by radical polymerization and thencontacting the fluoropolymer with fluorine gas, a step of converting the—SO₂F groups to sulfonic acid groups, and a step of forming the abovefluoropolymer into a membrane-form.

BEST MODE FOR CARRYING OUT THE INVENTION

The electrolyte material for polymer electrolyte fuel cells to beobtained by the present invention is a fluoropolymer having alicyclicstructures in its main chain and further having sulfonic acid groups,and is one highly fluorinated by contacting it with fluorine gas(hereinafter referred to as the present polymer). The present polymer isa polymer excellent in oxygen gas permeability and oxygen gassolubility, and its water discharging property is improved asfluorinated with fluorine gas.

The present inventors considered that since a linear perfluoropolymerhaving sulfonic acid groups which has been commonly used for fuel cells,has unstable functional groups such as —COOH groups, —CF═CF₂ groups,—COF groups and CF₂H groups at same molecular chain terminals, such apolymer gradually decomposes during long-term operation when used for anelectrolyte material for polymer electrolyte fuel cells, whereby thepower generation voltage decreases and the membrane strength decreasesto locally cause pinholes, breaking, abrasion or the like, and they havefound that the durability can be greatly improved by fluorinating(contacting with fluorine gas) such a polymer so as to stabilize themolecule terminals by perfluorination. However, in a case where thepolymer was exposed to severe operation conditions, such durability wasnot good enough. Accordingly, they have conducted a further study forimprovement of the durability, and as a result, have found that thedurability can be remarkably improved by fluorinating a polymer havingalicyclic structures in its main chain and further having sulfonic acidgroups, as compared with the durability improved by fluorinating theconventional polymer.

The present polymer is usually prepared by synthesizing a fluoropolymerhaving alicyclic structures in its main chain and further having —SO₂Fgroups, followed by hydrolysis and conversion to an acid form. Withregard to contacting it with fluorine gas, the fluoropolymer having the—SO₂F group may be hydrolyzed and converted to an acid form in advance,followed by fluorination with fluorine gas, but such fluorination ispreferably carried out in the step of a —SO₂F group (a precursor for asulfonic acid group) before the hydrolysis and conversion to an acidform, as the process will be easy. The fluorine gas to be used forfluorination is usually one diluted with an inert gas such as nitrogen,helium or carbon dioxide to a concentration of at least 0.1% and lessthan 100%, but may be used without dilution. The polymer can becontacted with the fluorine gas in the state of bulking or as dispersedor dissolved in a fluorosolvent.

The polymer obtained by polymerization may be fluorinated as it is, butmay be subjected to heat treatment before fluorination, so as to removevolatile components sufficiently or thermally stabilize the polymer. Insuch a case, the temperature is preferably from 200 to 300° C. in air orin an inert gas atmosphere such as nitrogen gas or under reducedpressure.

The temperature for the fluorination of the polymer by contact with thefluorine gas is usually from room temperature to 300° C., preferablyfrom 25 to 250° C., particularly preferably from 100 to 220° C., morepreferably from 150 to 200° C. If such a temperature is too low, thereaction of fluorine gas and polymer ends becomes slow, and if it is toohigh, —SO₂F groups tend to be detached. The contact time in the abovetemperature range is preferably from 1 minute to 1 week, particularlypreferably from 1 to 50 hours.

In the fluorination process, when the polymer is fluorinated asdispersed or dissolved in a fluorosolvent, the following solvents may,for example, be used as the fluorosolvent.

A polyfluorotrialkylamine compound such as perfluorotributylamine orperfluorotripropylamine.

A fluoroalkane such as perfluorohexane, perfluorooctane,perfluorodecane, perfluorododecane, perfluoro(2,7-dimethyloctane),2H,3H-perfluoropentane, 1H-perfluorohexane, 1H-perfluorooctane,1H-perfluorodecane, 1H,4H-perfluorobutane,1H,1H,1H,2H,2H-perfluorohexane, 1H,1H,1H,2H,2H-perfluorooctane,1H,1H,1H,2H,2H-perfluorodecane, 3H,4H-perfluoro(2-methylpentane) or2H,3H-perfluoro(2-methylpentane).

A chlorofluoroalkane such as 3,3-dichloro-1,1,1,2,2-pentafluoropropane,1,3-dichloro-1,1,2,2,3-pentafluoropropane or1,1-dichloro-1-fluoroethane.

A polyfluorocycloalkane such as perfluorodecalin, perfluorocyclohexane,perfluoro(1,2-dimethylcyclohexane), perfluoro(1,3-dimethylcyclohexane),perfluoro(1,3,5-trimethylcyclohexane) or perfluorodimethylcyclobutane(irrespective of the structural isomerism).

A hydrofluoroether such as n-C₃F₇OCH₃, n-C₃F₇OCH₂CF₃, n-C₃F₇OCHFCF₃,n-C₃F₇OC₂H₅, n-C₄F₉OCH₃, iso-C₄F₉OCH₃, n-C₄F₉OC₂H₅, iso-C₄F₉OC₂H₅,n-C₄F₉OCH₂CF₃, n-C₅F₁₁OCH₃, n-C₆F₁₃OCH₃, n-C₅F₁₁OC₂H₅,CF₃OCF(CF₃)CF₂OCH₃, CF₃OCHFCH₂OCH₃, CF₃OCHFCH₂OC₂H₅ orn-C₃F₇OCF₂CF(CF₃)OCHFCF₃, a fluorine-containing low molecular weightpolyether, an oligomer of chlorotrifluoroethylene or the like.

These solvents may be used alone or in combination as a mixture of twoor more of them.

In addition, many other compounds may be used. A chlorofluorocarbonsolvent such as 1,1,2-trichloro-1,2,2-trifluoroethane,1,1,1-trichloro-2,2,2-trifluoroethane,1,1,1,3-tetrachloro-2,2,3,3-tetrafluoropropane or1,1,3,4-tetrachloro-1,2,2,3,4,4-hexafluorobutane may be technicallyuseful, but is not preferred from the viewpoint of the globalenvironment protection. Further, it is also possible to carry out thereaction by means of liquid or supercritical carbon dioxide.

Among the above solvents, a solvent having hydrogen atoms will reactwith fluorine gas. Therefore, it is preferred to use a solvent having nohydrogen atoms.

The —SO₂F groups in the polymer fluorinated as described above will behydrolyzed in a solution of an alkali such as NaOH or KOH, or a mixedsolvent of water and a polar solvent such as an alcohol such as methanolor ethanol, or dimethylsulfoxide, and then will be converted to an acidform by e.g. hydrochloric acid or sulfuric acid, i.e. to sulfonic acidgroups. For example, when the polymer is hydrolyzed by an aqueous KOHsolution, —SO₂F groups will be converted to —SO₃K groups, and then Kions will be substituted by protons. The hydrolysis and conversion to anacid form are carried out usually at a temperature of from 0° C. to 120°C.

The polymer having sulfonic acid groups or —SO₂F groups as theirprecursors to be reacted with fluorine gas can be synthesized via aprocess of copolymerizing a monomer having cyclic structures or acyclopolymerizable monomer with a monomer having sulfonic acid groups orprecursors for the sulfonic acid groups. The above polymer is preferablya perfluoropolymer obtainable by copolymerizing only perfluoromonomerswhen the durability as an electrolyte material for fuel cells andeasiness of the fluorination process are taken into consideration. Amongthe perfluoropolymers, if —COF groups, —COOH groups, —CF═CF₂ groups andthe like are present at the terminals of the polymer main chain due toe.g. a chain transfer reaction and a polymerization initiator withhydrogen atoms is employed, non-perfluoro groups based on thepolymerization initiator will be produced at the terminals of thepolymer main chain. Therefore, the effect of fluorination will beobtained via the fluorination process.

When a perfluoro compound such as perfluorodiacyl peroxide representedby perfluorobutanoyl peroxide, is used as the above polymerizationinitiator, there may be a case where stable perfluoro groups will beintroduced to the terminals, and the unstable terminal groups willdecrease after the polymerization. If such a polymer having the unstableterminal groups decreased is further treated with fluorine gas, apolymer having very few unstable terminal groups can be more easilyobtained, such being preferred.

The cyclic structures in the present polymer are not particularlylimited, but are preferably cyclic structures represented by e.g. thefollowing formulae, wherein n is an integer of from 1 to 4, R^(f) is aC₁₋₈ perfluoroalkyl group or a perfluoroalkoxy group, and X and Y areeach independently a fluorine atom or a trifluoromethyl group. Eachcyclic structure is preferably one of 4- to 7-membered rings, and ispreferably a 5-membered ring or a 6-membered ring in consideration ofthe stability of the ring;

The monomer having a cyclic structure as a comonomer to obtain thepresent polymer may, for example, be perfluoro(2,2-dimethyl-1,3-dioxole)(hereinafter referred to PDD), perfluoro(1,3-dioxole),perfluoro(2-methylene-4-methyl-1,3-dioxolane) (hereinafter referred toMMD) or 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole.

The cyclopolymerizable monomer as a comonomer to obtain the presentpolymer may, for example, be perfluoro(3-butenylvinyl ether)(hereinafter referred to BVE), perfluoro[(1-methyl-3-butenyl)vinylether], perfluoro(allylvinyl ether) or1,1′-[(difluoromethylene)bis(oxy)]bis[1,2,2,-trifluoroethene].

With respect to specific examples of repeating units based on theabove-mentioned monomer having the above cyclic structure or acyclopolymerizable monomer, a repeating unit based on PDD may berepresented by the formula A, a repeating unit based on BVE may berepresented by the formula B, and a repeating unit based on MMD may berepresented by the formula C. In the present specification, “afluoropolymer having alicyclic structures” represents a fluoropolymercomprising repeating units having such cyclic structures containing nounsaturated bonds.

A monomer having a sulfonic acid group or a precursor group for thesulfonic acid group to be reacted with a monomer having a cyclicstructure or a cyclopolymerizable monomer is preferably a perfluorovinylether having a —SO₂F group. Specifically, a perfluorovinyl etherrepresented by CF₂═CF—(OCF₂CFY)_(m)—O_(p)—(CF₂)_(n)—SO₂F (wherein, Y isa fluorine atom or a trifluoromethyl group, m is an integer of from 0 to3, n is an integer of from 1 to 12, p is 0 or 1 and m+p>0) is preferred.Among such perfluorovinyl ethers, compounds of the formulae 1 to 3 arepreferred. Here, in the formulae 1 to 3, q is an integer of from 1 to 8,r is an integer of from 1 to 8, and s is 2 or 3. In the case ofpolymerization by using monomers having —SO₂F groups, hydrolysis andacid form-conversion treatment to convert them to —SO₃H groups arecarried out to form an electrolyte material. Namely, the present polymerfor the electrolyte material preferably contains repeating units basedon CF₂═CF—(OCF₂CFY)_(m)—O_(p)—(CF₂)_(n)—SO₃H.CF₂═CFO(CF₂)_(q)SO₂F  Formula 1CF₂═CFOCF₂CF(CF₃)O(CF₂)_(r)SO₂F  Formula 2CF₂═CF(OCF₂CF(CF₃))_(s)O(CF₂)₂SO₂F  Formula 3

Especially, the present polymer is preferably a polymer which containsrepeating units based on a monomer selected from the group consisting ofperfluoro(3-butenyl vinyl ether), perfluoro(2,2-dimethyl-1,3-dioxole),perfluoro(1,3-dioxole), 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxoleand perfluoro(2-methylene-4-methyl-1,3-dioxolane), and repeating unitsbased on perfluoro(3,6-dioxa-4-methyl-7-octene)sulfonic acid(CF₂═CFOCF₂CF(CF₃)O(CF₂)₂SO₃H) or perfluoro(3-oxa-4-pentene)sulfonicacid (CF₂═CFO(CF₂)₂SO₃H).

The present polymer is prepared via a step of copolymerizing theabove-mentioned cyclic monomer or the cyclopolymerizable monomer with amonomer having a sulfonic acid group or a precursor group for thesulfonic acid group represented by e.g. formulae 1 to 3, but othermonomers such as tetrafluoroethylene may further be copolymerized, e.g.to adjust the strength. Because if the present polymer is constitutedsolely by repeating units based on the monomer having a cyclic structureand the repeating units based on the monomer having a sulfonic acidgroup, its skeleton tends to be stiff, and in a case where such apolymer is used for a membrane or a catalyst layer for fuel cells, themembrane or the catalyst layer tends to be brittle.

Here, the present polymer is excellent in water repellency by thefluorination process after polymerization, and improves the output powerof a fuel cell and shows stable properties for a long term, when used asan electrolyte for a cathode of the fuel cell. However, in the case ofcopolymerizing other monomers, the content of the repeating units basedon such other monomers in the present polymer is preferably at most 35%,particularly preferably at most 20%, by mass ratio, so as not to impairsuch excellent output power characteristics.

The above-mentioned copolymerizable monomer may, for example, betetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride,hexafluoropropylene, trifluoroethylene, vinyl fluoride or ethylene.Further, a compound represented by CF₂═CFOR^(f1), CH₂═CHR^(f2),CH₂═CHCH₂R^(f2) or CF₂═CFOR^(f3)Z may also be used. Here, R^(f1)represents a C₁₋₁₂ perfluoroalkyl group which may be branched and maycontain an oxygen atom of an ether bond type. R^(f2) represents a C₁₋₁₂perfluoroalkyl group. R^(f3) represents a C₂₋₆ perfluoroalkylene groupwhich may be branched and may contain an oxygen atom of an ether bondtype. Z is —CN, —COOR (wherein R is a C₁₋₆ alkyl group) or —COF.

Among the above monomers, a perfluoromonomer is preferably used from theviewpoint of easiness of the reaction with fluorine gas and durability.Tetrafluoroethylene is particularly preferred, since it is readilyavailable and has a high polymerization reactivity.

The compound represented by CF₂═CFOR^(f1) among the above monomers ispreferably a perfluorovinyl ether compound represented byCF₂═CF—(OCF₂CFX)_(t)—O—R^(f4). Here, in the formula, t is an integer offrom 0 to 3, X is a fluorine atom or a trifluoromethyl group, and R^(f4)is a linear or branched C₁₋₁₂ perfluoroalkyl group (hereinafter, R^(f4)means the same in the present specification). The compounds representedby the formulae 4 to 6 are particularly preferred. Here, in theformulae, v is an integer of from 1 to 8, w is an integer of from 1 to8, and x is 2 or 3.CF₂═CFO(CF₂)_(v)CF₃  Formula 4CF₂═CFOCF₂CF(CF₃)O(CF₂)_(w)CF₃  Formula 5CF₂═CF(OCF₂CF(CF₃))_(x)O(CF₂)₂CF₃  Formula 6

In order to increase the strength of the membrane or the catalyst layerusing the present polymer, the number average molecular weight of thepresent polymer is preferably at least 5,000, more preferably at least10,000, further preferably at least 20,000. However, if the molecularweight is too high, the moldability or the solubility in theafter-mentioned solvent will decrease, and therefore, the molecularweight is preferably at most 5,000,000, more preferably at most2,000,000.

In the present polymer, the content of the repeating units based on themonomer having a cyclic structure is preferably from 0.5 to 80 mol %,more preferably from 1 to 80 mol %, further preferably from 4 to 70 mol%, and still further preferably from 10 to 70 mol %.

Even if the content of the repeating units based on the monomer having acyclic structure is small, the durability improves. However if it isless than 0.5%, the durability may tend to hardly improve. Further, ifthe content of the repeating units having cyclic structures is toolarge, the sulfonic acid groups in the polymer will decrease, and theelectroconductivity will decrease because the ion exchange capacitytends to be small.

Further, the repeating units having sulfonic acid groups are preferablycontained in such an amount that the ion exchange capacity of thepresent polymer will be from 0.5 to 2 meq/g dry resin, and are morepreferably contained in such an amount that it will be from 0.7 to 1.5meq/g dry resin. If such an ion exchange capacity is too low, theelectroconductivity of the polymer as an electrolyte material willdecrease, and if it is too high, the water repellency will deteriorate,and the durability will deteriorate when used for fuel cells, and thepolymer strength will be insufficient.

For the polymerization to obtain the present polymer, conventional knownmethods such as bulk polymerization, solution polymerization, suspensionpolymerization or emulsion polymerization may be carried out. Thepolymerization is carried out under a condition where radicals will begenerated, and a method of irradiating a radiation such as ultravioletrays, γ-rays or electron rays and a method of adding a radical initiatorto be used in usual radical polymerization, may be employed. Thepolymerization temperature is usually from about 20 to 150° C. Theradical initiator may, for example, be a bis(fluoroacyl)peroxide, abis(chlorofluoroacyl)peroxide, a dialkylperoxydicarbonate, a diacylperoxide, a peroxyester, an azo compound or a persulfate.

In solution polymerization, a solvent to be used usually has a boilingpoint of from 20 to 350° C., preferably from 40 to 150° C. from theviewpoint of handling efficiency. A useful solvent may be the samesolvent as the fluorosolvent exemplified as a suitable fluorosolventwhen the present polymer is fluorinated in the fluorosolvent. Namely, apolyfluorotrialkylamine compound, a perfluoroalkane, ahydrofluoroalkane, a chlorofluoroalkane, a fluoroolefin having no doublebond at the terminal of the molecular chain, a polyfluorocycloalkane, apolyfluorocyclic ether compound, a hydrofluoroether, afluorine-containing low molecular weight polyether or t-butanol may, forexample, be mentioned. These solvents may be used alone or incombination as a mixture of two or more of them. Further, it is alsopossible to carry out the polymerization by using liquid orsupercritical carbon dioxide.

The present polymer can be dissolved or dispersed suitably in a —OHgroup-containing organic solvent. Such a solvent is preferably analcoholic —OH group-containing organic solvent. Specifically, methanol,ethanol, 1-propanol, 2,2,2-trifluoroethanol,2,2,3,3,3-pentafluoro-1-propanol, 2,2,3,3-tetrafluoro-1-propanol,4,4,5,5,5-pentafluoro-1-pentanol, 1,1,1,3,3,3-hexafluoro-2-propanol,3,3,3-trifluoro-1-propanol, 3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol or3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol may, for example, bementioned. Further, as an organic solvent other than an alcohol, anorganic solvent having a carboxyl group such as acetic acid, may also beused, but is not restricted thereto.

The —OH group-containing organic solvents may be used as a mixture of aplurality of such solvents, or may be used as mixed with water or withother fluorosolvents. As such other fluorosolvents, the same solventsmay be exemplified as the fluorosolvents exemplified as preferredfluorosolvents when the present polymer is fluorinated in thefluorosolvent. When a mixed solvent is used, the content of the —OHgroup-containing organic solvent is preferably at least 10%,particularly preferably at least 20%, based on the total mass of thesolvents.

In such a case, the present polymer may be dissolved or dispersed in themixed solvent from the beginning. Otherwise, firstly, the presentpolymer may be dissolved or dispersed in the —OH group-containingorganic solvent, and then, other solvents may be mixed thereto.

The dissolution or the dispersion is preferably carried out within atemperature range of from 0 to 250° C., particularly preferably within arange of from 20 to 150° C. under atmospheric pressure or under such acondition as closed and pressurized by e.g. an autoclave.

Further, the present polymer may be dissolved or dispersed in analcoholic solvent having a boiling point lower than that of water, andthen water may be added and the alcohol may be distilled off to preparean aqueous dispersion containing substantially no organic solvent.

If a liquid composition obtainable by dissolving or dispersing thepresent polymer as described above, is used to prepare a cathode of thepolymer electrolyte fuel cell, it is possible to obtain a cathode whichis excellent in gas diffusibility and water repellency. Theconcentration of the present polymer in the liquid composition ispreferably from 1 to 50%, particularly preferably from 3 to 30%, basedon the total mass of the liquid composition. If the concentration is toolow, for example, a large amount of the organic solvent is required atthe time of preparing the cathode. If the concentration is too high, theviscosity of the liquid tends to be high, whereby handling tends to bedifficult.

In the present invention, for example, an electroconductive carbon blackpowder having platinum catalyst particles supported thereon is mixed anddispersed to the liquid composition containing the present polymer, andthe resulting uniform dispersion is used to obtain a membrane-electrodeassembly for polymer electrolyte fuel cells by either one of thefollowing two methods. A first method comprises applying the abovedispersion on both sides of a cation exchange membrane as amembrane-form polymer electrolyte and drying the dispersion, and thenbonding carbon cloth or carbon paper thereto. A second method comprisesapplying the above dispersion on carbon cloth or carbon paper and dryingthe dispersion, followed by bonding onto a cation exchange membrane.

In the polymer electrolyte fuel cell of the present invention, the massratio of the catalyst to the ion exchange resin as an electrolytematerial contained in the cathode, is preferably such that catalyst:ionexchange resin=40:60 to 95:5 from the viewpoint of electroconductivityof the electrode and water discharging property. The mass of thecatalyst here includes the mass of a carrier in a case of a supportedcatalyst in which the catalyst is supported on a carrier such as carbon.

Further, the ion exchange resin in the cathode may be a resin made ofthe present polymer alone, or may be a mixture of a conventional knownperfluoropolymer having sulfonic acid groups and the present polymer.The conventional known polymer may, for example, be a copolymer oftetrafluoroethylene and a perfluorovinyl ether represented byCF₂═CF—(OCF₂CFY)_(m)—O_(p)—(CF₂)_(n)—SO₃H (wherein Y is a fluorine atomor a trifluoromethyl group, m is an integer of from 0 to 3, n is aninteger of from 1 to 12, p is 0 or 1, and m+p>0). Particularly preferredis a polymer having sulfonic acid groups, obtained by hydrolysis and aconversion into an acid form of a copolymer of tetrafluoroethylene and amonomer represented by any one of the above formulae 1 to 3.

In the case of a cathode wherein the conventional known polymer is usedas mixed with the present polymer, the ratio of the present polymer ispreferably at least 20%, particularly preferably at least 50%, based onthe total mass of the ion exchange resins in the cathode.

The anode in the present invention may be the same as the cathode, ormay be made of e.g. a gas diffusion electrode which is used heretofore.The anode may be formed by the same process as the cathode, whereby amembrane-electrode assembly having an anode disposed on one side of themembrane and a cathode disposed on another side can be obtained. Thepresent polymer is an electrolyte material for polymer electrolyte fuelcells, and such a polymer may be contained in the anode instead of inthe cathode, or may be used as a material for an ion exchange membraneas a membrane-form polymer electrolyte.

The resulting membrane-electrode assembly may, for example, beinterposed between separators made of e.g. an electroconductive carbonplate having grooves formed as channels for a fuel gas or an oxidant gas(air, oxygen and the like) containing oxygen, and then assembled in acell to obtain the polymer electrolyte fuel cell of the presentinvention. The polymer electrolyte fuel cell using the electrolytematerial of the present invention is not restricted to a hydrogen-oxygenfuel cell, and is applicable to a direct methanol fuel cell (DMFC) orthe like. Also in such a case, the present polymer is preferablycontained in the cathode.

Now, the present invention will be described in detail with reference toExamples. However, it should be understood that the present invention isby no means restricted thereto.

In the following Examples, the following abbreviations are used.

PSVE: CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F,

PSVE2: CF₂═CFOCF₂CF₂OCF₂CF₂SO₂F,

IPP: (CH₃)₂CHOC(═O)OOC(═O)OCH(CH₃)₂,

PFB: CF₃CF₂CF₂C(═O)OOC(═O)CF₂CF₂CF₃,

HCFC 141b: CH₃CCl₂F (manufactured by Asahi Glass Company, Limited),

HCFC 225cb: CClF₂CF₂CHClF (manufactured by Asahi Glass Company, Limited)

Example 1 Preparation of PDD/PSVE Copolymer

Into an autoclave having a capacity of 200 ml, 26.0 g of PDD, 127.8 g ofPSVE and 0.46 g of IPP were put, and after deaeration, nitrogen wasintroduced so that the pressure would be 0.3 MPa. Then, the temperaturewas raised to 40° C., and polymerization was initiated with stirring.After 10 hours, the interior of the autoclave was cooled, and purge wascarried out to stop the polymerization. After diluting with HCFC 225cb,the resulting mixture was poured into hexane to precipitate the polymer,which was washed twice with hexane and further washed once with HCFC141b. After filtration, vacuum drying was carried out at 80° C. for 16hours to obtain 41.6 g of a white polymer. The content of sulfur wasdetermined by an elemental analysis, and the ratio of PDD/PSVE and theion exchange capacity were determined and formed to be 56.5/43.5 (molarratio), and 1.31 meq/g dry resin, respectively. Further, the molecularweight was measured by GPC, whereby the number average molecular weightas calculated as polymethyl methacrylate was 33,000.

Into a Hastelloy autoclave of 2,000 ml, 10 g of the above polymer wasput, and after deaeration, fluorine gas (20 vol %) diluted with nitrogengas was introduced so that the gauge pressure would be 0.3 MPa. Thereaction system was maintained at 180° C. for 4 hours. Then, theresulting polymer was hydrolyzed with an alkali, converted to an acidform and dried, and then dissolved in ethanol to obtain a transparent10% solution. A cast film having a thickness of 200 μm was prepared fromthis solution, and heated at 160° C. for 30 minutes. Then, the resultingcast film was set in TMA (manufactured by Mac Science Company). The castfilm was heated at a rate of 5° C./min under a load of 3.5 g exerted bya quartz probe having a diameter of 1 mm. The temperature, at which thethickness of the film started to abruptly decrease due to penetration ofthe probe into the cast film, was measured as the softening temperature.As a result, the softening temperature of this polymer was 150° C.

Example 2 Preparation of BVE/PSVE Copolymer

In a nitrogen atmosphere, 120.0 g of BVE, 128.5 g of PSVE and 0.76 g ofIPP were put into a flask of 300 ml, and the temperature in the flaskwas raised to 40° C. to initiate polymerization with stirring. After16.7 hours, the polymerization was stopped, and the product was put intohexane to precipitate the polymer, which was further washed three timeswith hexane. After filtration, vacuum drying was carried out at 80° C.for 16 hours to obtain 47.8 g of a white polymer.

The content of sulfur was obtained by an elemental analysis, and theratio of BVE/PSVE and the ion exchange capacity were obtained, wherebythey were respectively BVE/PSVE=67.0/33.0 (molar ratio) and 0.99 meq/gdry resin. Further, the molecular weight was measured by GPC, wherebythe number average molecular weight as calculated as polymethylmethacrylate was 29,000. Into a Hastelloy autoclave of 2,000 ml, 10 g ofthe above polymer was put, and after deaeration, fluorine gas (20 vol %)diluted with nitrogen gas was introduced so that the gauge pressurewould be 0.3 MPa. The reaction system was maintained at 180° C. for 4hours. Then, the resulting polymer was hydrolyzed with an alkali,converted to an acid form and dried, and then dissolved in ethanol toobtain a transparent 10% solution. The softening temperature of thispolymer measured in the same manner as in Example 1 was 110° C.

Example 3 Preparation of TFE/PDD/PSVE Copolymer

Into an autoclave having a capacity of 200 ml, 14.30 g of PDD, 52.64 gof PSVE, 76.94 g of HCFC 225cb and 0.36 g of IPP were put, followed byfreeze-deaeration. After introducing 5.9 g of TFE, the temperature wasraised to 40° C. to initiate polymerization. The pressure at that timewas 0.26 MPa (gauge pressure). The reaction was carried out at 40° C.for 10 hours, and when the pressure became 0.07 MPa (gauge pressure),the reaction was terminated. The polymerization solution was dilutedwith HCFC 225cb and subjected to flocculation with hexane, followed bywashing three times with hexane. Vacuum drying was carried out at 80° C.overnight to obtain 25.03 g of a polymer (yield: 34.4%).

The composition of the polymer was determined by ¹⁹F-NMR, wherebyTFE/PDD/PSVE=42/35/22 (molar ratio), and the ion exchange capacity was0.98 meq/g dry resin. Further, the number average molecular weight ascalculated as polymethyl methacrylate, by GPC, was 53,000 and the weightaverage molecular weight was 83,000. 10 g of this polymer was put into aHastelloy autoclave of 2,000 ml, followed by deaeration. Fluorine gas(20 vol %) diluted with nitrogen gas was introduced so that the gaugepressure would be 0.3 MPa, and the system maintained at 180° C. for 4hours. Then, the resulting polymer was hydrolyzed with an alkali,converted to an acid form and dried, and then dissolved in ethanol toobtain a transparent 12% solution.

Further, by using the polymer obtained by the treatment with fluorinegas, hot pressing was applied to prepare a film having a thickness of100 μm. This film was immersed in a solution of KOH/H₂O/DMSO=11/59/30(mass ratio) and maintained at 90° C. for 17 hours for hydrolysis. Then,after cooling to room temperature, the film was washed three times withwater. Further, it was immersed in 2N sulfuric acid at room temperaturefor 2 hours and then washed with water. This immersion in sulfuric acidand washing with water were carried out in a total of three times each,and finally washing with water was carried out three times. Air-dryingat 80° C. for 16 hours was carried out, and further, vacuum drying at80° C. was carried out to obtain an acid-form converted dry film. Thedynamic viscoelasticity was measured, and the temperature at which themodulus abruptly decreased was obtained as a softening temperature. As aresult, the softening temperature of this polymer was 120° C.

Example 4 Preparation of TFE/PDD/PSVE Copolymer 2

Using 9 g of TFE, 24.4 g of PDD, 102.6 g of PSVE and 0.08 g of IPP,polymerization was carried out in the same manner as in Example 3 exceptthat HCFC 225cb was not used. Polymerization was carried out at 40° C.for 12 hours to complete the reaction. The resulting polymerizationsolution was diluted with HCFC 225cb and subjected to flocculation withhexane, followed by washing three times with hexane. Vacuum drying wascarried out at 80° C. overnight to obtain 37.8 g of a polymer (yield:27.7%).

The molecular weight and composition were measured in the same manner asin Example 3. The composition of the resulting polymer wasTFE/PDD/PSVE=36/41/23, and the ion exchange capacity was 0.97 meq/g dryresin. Further, the number average molecular weight was 160,000, and theweight average molecular weight was 280,000. The resulting polymer wassubjected to heat treatment under reduced pressure at 240° C. for 4hours and then treated with fluorine gas in the same manner as inExample 3.

Example 5 Preparation of TFE/PDD/PSVE2 Copolymer

Using 6 g of TFE, 16.5 g of PDD, 68.3 g of PSVE2 and 0.05 g of IPP,polymerization was carried out in the same manner as in Example 4.Polymerization was carried out at 40° C. for 20 hours. The resultingpolymerization solution was diluted with HCFC 225cb and subjected toflocculation with hexane, followed by washing three times with hexane.Vacuum drying was carried out at 80° C. overnight to obtain 27.3 g apolymer (yield: 30.1%).

The molecular weight and composition were measured and fluorine gastreatment was carried out in the same manner as in Example 3. Thecomposition of the resulting polymer was TFE/PDD/PSVE2=36/39/26, and theion exchange capacity was 1.11 meq/g dry resin. Further, the numberaverage molecular weight was 167,000, and the weight average molecularweight was 870,000.

Example 6 Preparation of TFE/MMD/PSVE Copolymer

Into an autoclave having a capacity of 200 ml, 14.1 g of MMD, 78.0 g ofPSVE and 0.3 g of HCFC 225cb solution containing 3 mass % PFB were put,followed by freeze-deaeration. After introducing 14.1 g of TFE,polymerization was carried out at 20° C. for 22 hours. Thepolymerization solution was diluted with HCFC 225cb and subjected toflocculation with hexane, followed by washing three times with hexane.Vacuum drying was carried out at 80° C. overnight to obtain 2.2 g of apolymer.

With regard to the resulting polymer, the molecular weight andcompositions were measured and fluorine gas treatment was carried out inthe same manner as in Example 3. The composition of the resultingpolymer was TFE/MMD/PSVE=30/47/23 (molar ratio), and the ion exchangecapacity was 0.93 meq/g dry resin. Further, the number average molecularweight as calculated as polymethyl methacrylate measured, by GPC was155,000, and the weight average molecular weight was 239,000.

Example 7 Preparation of TFE/MMD/PSVE Copolymer 2

Into an autoclave having a capacity of 200 ml, 0.7 g of MMD, 92.6 g ofPSVE, 50.8 g of HCFC 225cb and 2.57 g of HCFC 225cb solution containinga 3 mass % PFB were put, followed by freeze-deaeration. The temperaturewas raised to 40° C., and TFE was introduced so that the gauge pressurewould be 0.5 MPa. Then, TFE was introduced while this pressure wasmaintained, and polymerization was carried out at 40° C. for 7 hours.The polymerization solution was subjected to flocculation with HCFC141b, followed by washing three times with HCFC 141b. Vacuum drying wascarried out at 80° C. overnight to obtain 19.9 g of a polymer.

The resulting polymer was hydrolyzed in a KOH aqueous solution, andtitrated with an aqueous hydrochloric acid, whereby the ion exchangecapacity was 1.06 meq/g dry resin. Further, the composition of thepolymer was determined by ¹⁹F-NMR, whereby TFE/MMD/PSVE=77/5/18 (molarratio). This polymer was treated with fluorine gas in the same manner asin Example 3.

Example 8 Preparation of TFE/MMD/PSVE Copolymer 3

Into an autoclave having a capacity of 200 ml, 0.4 g of MMD, 93.0 g ofPSVE, 53.3 g of HCFC 225cb and 2.62 g of a HCFC 225cb solutioncontaining a 3 mass % PFB were put, followed by freeze-deaeration. Thetemperature was raised to 40° C., and TFE was introduced so that thegauge pressure would be 0.45 MPa. Then, TFE was introduced while thispressure was maintained, and polymerization was carried out at 40° C.for 7 hours. The polymerization solution was subjected to flocculationwith HCFC 141b, followed by washing three times with HCFC 141b. Vacuumdrying was carried out at 80° C. overnight to obtain 16.7 g of apolymer.

The resulting polymer was hydrolyzed in a KOH aqueous solution, andtitrated with an aqueous hydrochloric acid, whereby the ion exchangecapacity was 1.04 meq/g dry resin. Further, the composition of thepolymer was determined by ¹⁹F-NMR, whereby TFE/MMD/PSVE=74/8/18 (molarratio). This polymer was treated with fluorine gas in the same manner asin Example 3.

Example 9 TFE/MMD/PSVE 4

Into an autoclave having a capacity of 200 ml, 2.4 g of MMD, 91.8 g ofPSVE, 55.2 g of HCFC 225cb and 2.66 g of a HCFC 225cb solutioncontaining a 3 mass % PFB were put, followed by freeze-deaeration. Thetemperature was raised to 40° C., and TFE was introduced so that thegauge pressure would be 0.40 MPa. Then, TFE was introduced while thispressure was maintained, and polymerization was carried out. Thepolymerization was carried out at 40° C. for 7 hours. The polymerizationsolution was subjected to flocculation with HCFC 141b, followed bywashing three times with HCFC 141b. Vacuum drying was carried out at 80°C. overnight to obtain 14.9 g of a polymer.

The resulting polymer was hydrolyzed in a KOH aqueous solution, andtitrated with an aqueous hydrochloric acid, whereby the ion exchangecapacity was 1.13 meq/g dry resin. Further, the composition of thepolymer was determined by ¹⁹F-NMR, whereby TFE/MMD/PSVE=61/16/23 (molarratio). This polymer was treated with fluorine gas in the same manner asin Example 3.

Example 10 Preparation of TFE/BVE/PSVE Copolymer

Into an autoclave having a capacity of 200 ml, 48.6 g of BVE, 86.4 g ofPSVE, 86.2 g of 1,1,2-trichlorotrifluoroethane and 0.75 g of a HCFC225cb solution containing a 3 mass % PFB were put, followed byfreeze-deaeration. The temperature was raised to 30° C., and TFE wasintroduced so that the pressure would be 0.15 MPa. Then, TFE wasintroduced while this pressure was maintained, and polymerization wascarried out. The polymerization was carried out at 30° C. for 16 hours.The polymerization solution was subjected to flocculation with hexane,followed by washing three times with hexane. Vacuum drying was carriedout at 80° C. overnight to obtain 8.3 g of a polymer:

The resulting polymer was hydrolyzed in a KOH aqueous solution, andtitrated with an aqueous hydrochloric acid, whereby the ion exchangecapacity was 0.95 meq/g dry resin. Further, the composition of thepolymer was measured by ¹⁹F-NMR, whereby TFE/BVE/PSVE=61/20/19 (molarratio). This polymer was treated with fluorine gas in the same manner asin Example 3.

Comparative Example 1

The polymer of Example 4 was recovered without treatment with fluorinegas. The composition and the molecular weight were the same.

Comparative Example 2

10 g of a powder of a copolymer comprising TFE and PSVE (ion exchangecapacity measured when converted into an acid form: 1.1 meq/g dry resin,hereinafter referred to as copolymer A) was subjected to heat treatmentin a reduced pressure oven under a pressure of 10 Pa, and at atemperature of 250° C. for 4 hours. Then, the treatment with fluorinegas was carried out in the same manner as in Example 3.

Preparation of Fuel Cell and Evaluation Test for Durability ofElectrolyte Material

A fuel cell was assembled as follows. A copolymer comprising repeatingunits based on CF₂═CF₂ and repeating units based onCF₂═CF—OCF₂CF(CF₃)O(CF₂)₂SO₃H (ion exchange capacity 1.1 meq/g dryresin) and a platinum-loaded carbon were mixed in a mass ratio of 1:3,and the mixture was mixed with ethanol to prepare a coating solution.This coating solution was applied on an ethylene-tetrafluoroethylenefilm substrate by die coating and dried to form a 10 μm thick electrodelayer having a platinum content of 0.5 mg/cm².

Then, each of the polymers obtained in Examples 4 to 10 and ComparativeExamples 1 and 2 was subjected to hot pressing to prepare a film havinga thickness of 50 μm. Each film was immersed in a solution ofKOH/H₂O/DMSO=15/55/30 (mass ratio) and maintained at 80° C. for 17 hoursfor hydrolysis. Then, after cooling to room temperature, it was washedthree times with water. Further, it was immersed in a 3 mol/Lhydrochloric acid at room temperature for 2 hours and then washed withwater. This immersion in hydrochloric acid and washing with water werecarried out in a total of three times each, and finally washing withwater was carried out for three times. Air-drying at 60° C. for 16 hourswas carried out to obtain an electrolyte membrane. Further, also withrespect to the copolymer A in Comparative Example 2 (withoutfluorination treatment), an electrolyte membrane was obtained in thesame manner.

Then, a pair of electrode layers obtained as mentioned above weredisposed so that the respective electrode layers were faced each otherand the electrolyte membrane was interposed therebetween, and pressingwas carried out to prepare one having the electrode layers transferredonto the membrane in each Example. Further, carbon cloths were put asgas diffusion layers on both sides to obtain a membrane-electrodeassembly.

Further, carbon plates having narrow zigzag grooves as gas channels cutby machining were put on both sides as separators, and then heaters weredisposed on the outer surfaces to assemble a polymer electrolyte fuelcell having an effective membrane surface area of 25 cm².

The durability was evaluated by the following method. The temperature ofthe fuel cell was maintained at 90° C. in a state where the circuit wasopened, steam-containing air was supplied to the cathode at a dew pointof 50° C., and steam-containing hydrogen was supplied to the anode at adew point of 50° C. at a rate of 50 ml/min, respectively. After theoperation was continued for the time shown in Table in such a state, thefuel cell was disassembled, and the deterioration state of theelectrolyte membrane was measured by mass measurement. The result isshown in Table. Here, the mass decreasing rate is calculated by dividingthe mass decrease (%) by the operation time (hr).

TABLE Operation Mass decrease Mass decreasing time (hr) (%) rate Example4 472 3 0.01 Example 5 125 3 0.02 Example 6 115 2 0.02 Example 7 117 10.01 Example 8 95 3 0.03 Example 9 139 1 0.01 Example 10 116 3 0.03Comparative 138 11 0.08 Example 1 Comparative 95 6 0.06 Example 2Copolymer A 95 20 0.21

INDUSTRIAL APPLICABILITY

The electrolyte material of the present invention is a sulfonic acidpolymer having a softening temperature higher than the softeningtemperature of 80° C. of atetrafluoroethylene/CF₂═CFCF₂CF(CF₃)O(CF₂)₂SO₃H copolymer, usedheretofore, as shown in Examples. The softening temperature isparticularly high, especially when it is a polymer copolymerized byusing a monomer having a cyclic structure or a cyclopolymerizablemonomer, of which the softening temperature of a homopolymer is at least100° C.

The modulus of the above conventional polymer starts to decreaseabruptly from near 80° C., and the softening temperature is close to theoperation temperature of a fuel cell, and therefore, the properties suchas swelling are likely to change with time, and there is a problem indurability if such a polymer is used as an electrolyte for a fuel cell.Further, it is difficult to operate the fuel cell at a temperature of atleast 80° C. Whereas, the electrolyte material of the present inventionhas a high softening temperature, and therefore, the properties do notchange with time and high durability can be achieved if it is used as apolymer for an electrolyte contained in an electrolyte membrane orelectrodes for a fuel cell. Further, it is also possible to operate thecell at a temperature higher than 80° C.

Further, the electrolyte material for polymer electrolyte fuel cells ofthe present invention, has alicyclic structures in its main chain, andthus has excellent gas diffusibility, and it is highly fluorinated andthus has excellent water repellency and excellent durability even in along-term operation of the electrolyte fuel cell.

The entire disclosure of Japanese Patent Application No. 2003-011097filed on Jan. 20, 2003 including specification, claims and summary isincorporated herein by reference in its entirety.

1. A process for producing a membrane-electrode assembly for polymerelectrolyte fuel cells which comprises a membrane-form polymerelectrolyte, a cathode disposed on one side of the electrolyte andcomprising a catalyst and a perfluoropolymer having alicyclic structuresin its main chain and further having sulfonic acid groups, and an anodedisposed on the other side of the electrolyte, wherein theperfluoropolymer having sulfonic acid groups is produced via a step ofobtaining a perfluoropolymer having alicyclic structures in its mainchain and further having —SO₂F groups by radical polymerization and thencontacting the perfluoropolymer with fluorine gas, and a step ofconverting the —SO₂F groups to the sulfonic acid groups.
 2. The processfor producing a membrane-electrode assembly for polymer electrolyte fuelcells according to claim 1, wherein the perfluoropolymer having sulfonicacid groups is made of a copolymer comprising repeating units based onthe following monomer A and repeating units based on the followingmonomer B (wherein Y is a fluorine atom or a trifluoromethyl group, m isan integer of from 0 to 3, p is 0 or 1, and n is an integer of from 1 to12): Monomer A: A perfluoromonomer which gives a polymer havingrepeating units containing cyclic structures in its main chain byradical polymerization Monomer B:CF₂═CF—(OCF₂CFY)_(m)—O_(p)—(CF₂)_(n)—SO₃H.
 3. The process for producinga membrane-electrode assembly for polymer electrolyte fuel cellsaccording to claim 2, wherein the monomer A is selected from the groupconsisting of perfluoro(3-butenyl vinyl ether),perfluoro(2,2-dimethyl-1,3-dioxole), perfluoro(1,3-dioxole),2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole andperfluoro(2-methylene-4-methyl-1,3-dioxolane), and the monomer B isperfluoro(3,6-dioxa-4-methyl-7-octene)sulfonic acid orperfluoro(3-oxa-4-pentene)sulfonic acid.
 4. The process for producing amembrane-electrode assembly for polymer electrolyte fuel cells accordingto claim 1, wherein said contacting the perfluoropolymer with fluorinegas is carried out at a temperature of from 150 to 200° C.
 5. Theprocess for producing a membrane-electrode assembly for polymerelectrolyte fuel cells according to claim 1, wherein said contacting theperfluoropolymer with fluorine gas is carried out over a period of timeof from 1 to 50 hours.
 6. The process for producing a membrane-electrodeassembly for polymer electrolyte fuel cells according to claim 1,wherein said contacting the perfluoropolymer with fluorine gas iscarried out over a period of time of from 1 to 50 hours at a temperatureof from 150 to 200° C.
 7. The process for producing a membrane-electrodeassembly for polymer electrolyte fuel cells according to claim 1,wherein the perfluoropolymer is subjected to heat treatment prior tocontacting the perfluoropolymer with fluorine gas.
 8. The process forproducing a membrane-electrode assembly for polymer electrolyte fuelcells according to claim 1, wherein the perfluoropolymer is dispersed ordissolved in a fluorosolvent having no hydrogen atoms prior tocontacting the perfluoropolymer with fluorine gas.